In its simplest form, a light-emitting diode (LED) is a light source composed of a p-n junction diode that emits light when activated. When a suitable voltage is applied to electrical contacts on opposing sides of the junction, electrons are able to recombine with holes within an active region of the device, releasing energy in the form of photons. The wavelengths of the emitted light and correspondingly, the color of the LED for visible wavelengths, is generally determined by the energy band gap of the semiconductor. LEDs are often small devices (e.g., less than 1 mm2) and integrated optical components may be used to shape the radiation pattern.
Generally, LEDs can be formed from inorganic or organic semiconductor materials. An inorganic LED (ILED) is formed using inorganic materials such as compound semiconductors (e.g., InGaAs, InGaN). Compared to organic LEDs (OLEDs) formed using organic materials, ILEDs are capable of generating light at a significantly higher power efficiency at a brightness per unit area that is several orders of magnitude higher (e.g., 1,000×-10,000×) than that of OLEDs.
Due to the high efficiency of ILED devices, extremely small inorganic LEDs can yield sufficient light to be practically useful for applications like displays or general illumination sources. LED dies having active areas of 100 μm2 or less and a thickness of 10 μm or less can generate light visible to the human eye at drive currents on the order of tens of nanoamps. Such devices are often referred to as microLEDs, mLEDs, or μEDs. Generally, microLEDs have an active area in a range from 1 μm2 to about 2500 μm2 and are manufactured using conventional semiconductor manufacturing techniques. As a result, microLEDs can have many different geometries, as specific applications demand.
As noted above, the wavelength emitted by an LED is primarily set by the material composition, e.g., the compound semiconductor forming the diode. Additionally, depositing multiple layers of the compound semiconductor is often difficult or infeasible for variety of reasons including manufacturing process complexity and cost considerations. Therefore, fabricating ILEDs emitting at different wavelengths (e.g., red, green, blue) on a common substrate (“monolithic integration”) to form an integrated device may be a challenge.